linux-kernel - Re: [PATCH v4 05/15] mm: introduce execmem_alloc() and execmem

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Date: Mon, 15 Apr 2024 18:36:39 +0100
From: Mark Rutland <mark.rutland@....com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Mike Rapoport <rppt@...nel.org>, linux-kernel@...r.kernel.org,
	Alexandre Ghiti <alexghiti@...osinc.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Bj\"orn T\"opel" <bjorn@...nel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Christophe Leroy <christophe.leroy@...roup.eu>,
	"David S. Miller" <davem@...emloft.net>,
	Dinh Nguyen <dinguyen@...nel.org>,
	Donald Dutile <ddutile@...hat.com>,
	Eric Chanudet <echanude@...hat.com>,
	Heiko Carstens <hca@...ux.ibm.com>, Helge Deller <deller@....de>,
	Huacai Chen <chenhuacai@...nel.org>,
	Kent Overstreet <kent.overstreet@...ux.dev>,
	Luis Chamberlain <mcgrof@...nel.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	Nadav Amit <nadav.amit@...il.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Puranjay Mohan <puranjay12@...il.com>,
	Rick Edgecombe <rick.p.edgecombe@...el.com>,
	Russell King <linux@...linux.org.uk>, Song Liu <song@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
	Thomas Gleixner <tglx@...utronix.de>, Will Deacon <will@...nel.org>,
	bpf@...r.kernel.org, linux-arch@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-mips@...r.kernel.org,
	linux-mm@...ck.org, linux-modules@...r.kernel.org,
	linux-parisc@...r.kernel.org, linux-riscv@...ts.infradead.org,
	linux-s390@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org, loongarch@...ts.linux.dev,
	netdev@...r.kernel.org, sparclinux@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH v4 05/15] mm: introduce execmem_alloc() and execmem_free()

On Mon, Apr 15, 2024 at 09:52:41AM +0200, Peter Zijlstra wrote:
> On Thu, Apr 11, 2024 at 07:00:41PM +0300, Mike Rapoport wrote:
> > +/**
> > + * enum execmem_type - types of executable memory ranges
> > + *
> > + * There are several subsystems that allocate executable memory.
> > + * Architectures define different restrictions on placement,
> > + * permissions, alignment and other parameters for memory that can be used
> > + * by these subsystems.
> > + * Types in this enum identify subsystems that allocate executable memory
> > + * and let architectures define parameters for ranges suitable for
> > + * allocations by each subsystem.
> > + *
> > + * @EXECMEM_DEFAULT: default parameters that would be used for types that
> > + * are not explcitly defined.
> > + * @EXECMEM_MODULE_TEXT: parameters for module text sections
> > + * @EXECMEM_KPROBES: parameters for kprobes
> > + * @EXECMEM_FTRACE: parameters for ftrace
> > + * @EXECMEM_BPF: parameters for BPF
> > + * @EXECMEM_TYPE_MAX:
> > + */
> > +enum execmem_type {
> > +	EXECMEM_DEFAULT,
> > +	EXECMEM_MODULE_TEXT = EXECMEM_DEFAULT,
> > +	EXECMEM_KPROBES,
> > +	EXECMEM_FTRACE,
> > +	EXECMEM_BPF,
> > +	EXECMEM_TYPE_MAX,
> > +};
> 
> Can we please get a break-down of how all these types are actually
> different from one another?
> 
> I'm thinking some platforms have a tiny immediate space (arm64 comes to
> mind) and has less strict placement constraints for some of them?

Yeah, and really I'd *much* rather deal with that in arch code, as I have said
several times.

For arm64 we have two bsaic restrictions: 

1) Direct branches can go +/-128M
   We can expand this range by having direct branches go to PLTs, at a
   performance cost.

2) PREL32 relocations can go +/-2G
   We cannot expand this further.

* We don't need to allocate memory for ftrace. We do not use trampolines.

* Kprobes XOL areas don't care about either of those; we don't place any
  PC-relative instructions in those. Maybe we want to in future.

* Modules care about both; we'd *prefer* to place them within +/-128M of all
  other kernel/module code, but if there's no space we can use PLTs and expand
  that to +/-2G. Since modules can refreence other modules, that ends up
  actually being halved, and modules have to fit within some 2G window that
  also covers the kernel.

* I'm not sure about BPF's requirements; it seems happy doing the same as
  modules.

So if we *must* use a common execmem allocator, what we'd reall want is our own
types, e.g.

	EXECMEM_ANYWHERE
	EXECMEM_NOPLT
	EXECMEM_PREL32

.. and then we use those in arch code to implement module_alloc() and friends.

Mark.